A ceramic capacitor, also referred to as a dielectric ceramic, is a capacitor in which a ceramic material acts as the dielectric and a metal as the electrode. Typically, a ceramic capacitor is constructed of two or more alternating layers of ceramic dielectric and metal. The composition of the ceramic material governs the electrical behaviors and, hence, applications of the resulting ceramic capacitor. Ceramic capacitors are commonly divided into two application classes: Class 1 ceramic capacitors offer high stability and low losses for resonant circuit applications and Class 2 ceramic capacitors offer high volume efficiency for buffer, by-pass and coupling applications.
The two major groups of ceramic materials utilized for ceramic capacitors are paraelectric (e.g. titanium dioxide) and ferroelectric material (e.g. barium titanate). A typical ceramic material as the dielectric phase is a mixture of a paraelectric material with an additive, a ferroelectric material with an additive, or a paraelectric material with a ferroelectric material. A paraelectric material-containing mixture can lead to a ceramic capacitor having a very stable and linear behavior of the capacitance value within a specified temperature range and low losses at high frequencies. However, titanium dioxide mixtures have a relatively low permittivity, implying that the capacitance value of the resulting capacitor is relatively small. Higher capacitance values for ceramic capacitors can be attained by using mixtures of ferroelectric materials, such as barium titanate, together with other metal oxides. These dielectric materials have much higher permittivity values, but their capacitance values are largely nonlinear over the temperature range, and losses at high frequencies are much higher. These different electrical characteristics of ceramic capacitors serve as bases for grouping them into “application classes”.
The dielectric material in a ceramic capacitor is typically composed of a mixture of finely ground particles of paraelectric or ferroelectric materials with other materials to achieve the desired characteristics. These powder mixtures are sintered at high temperatures to obtain the dielectric ceramic. The ceramic forms the dielectric (hence, the term dielectric ceramic), which serves as a substrate for the metallic electrodes. In the commercially available ceramic capacitors, the minimum thickness of the dielectric layer for low voltage capacitors is in the size range of 0.5 μm due to the usually large grain sizes of the ceramic powder. The thickness of the dielectric for capacitors with higher voltages is determined by the dielectric strength of the desired capacitor.
Commonly used ceramic capacitors come in various shapes and styles, including: (a) multilayer ceramic chip capacitor (MLCC), typically a rectangular block, for surface mounting; (b) ceramic disc capacitor (typically a single layer disc that is resin-coated, with through-hole leads); (c) feedthrough ceramic capacitor, used for bypass purposes in high-frequency circuits (e.g. tube shape, inner metallization contacted with a lead, outer metallization for soldering); (d) ceramic power capacitors, larger ceramic bodies in different shapes for high voltage applications.
An MLCC can be composed of a number of individual capacitors stacked together in parallel, which are contacted via the terminal surfaces. In other words, each MLCC includes a stack of internal planar electrodes arranged in parallel fashion with a ceramic layer there between, wherein the internal planar electrodes are of alternate polarity. External terminals, defined to be cathode or anode, are each in electrical contact with alternate internal electrodes.
Ceramic disc capacitors are commonly used for suppressing electromagnetic interference/radio frequency interference (EMI/RFI); e.g. for safety standard classes X1/Y2. Mainly due to their nonflammability in case of short circuit and their compatibility against high peak over-voltages (transient voltage), ceramic capacitors are often used as AC line filters for EMI or RFI suppression. These capacitors, also known as safety capacitors, are crucial components to reduce or suppress electrical noise caused by the operation of electrical or electronic equipment, while also providing limited protection against human endanger during short circuits.
Suppression capacitors are effective in reducing interference due to their electrical impedance decreasing with increasing frequency, so that at higher frequencies they short circuit electrical noise and transients between the lines, or ground. Hence, they can prevent equipment and devices (including motors, inverters, electronic ballasts, solid-state relay snubbers, and spark quenchers, etc.) from sending and receiving electromagnetic and radio frequency interference as well as transients in across-the-line (X capacitors) and line-to-ground (Y capacitors) connections. X capacitors effectively absorb symmetrical, balanced, or differential interference. Y capacitors are connected in a line bypass between a line phase and a point of zero potential, to absorb asymmetrical, unbalanced, or common-mode interference.
Large screen-size/high definition TVs and LCD displays are highly popular consumer electronic devices. Yet, smaller-sized computers (e.g. tablets) and telecommunications devices (smart phones) are becoming more popular. In both types of devices, a need exists for downsizing of the switching power supply circuits. Thus, the ceramic capacitors are required to be more compact and lighter in weight to meet the downsizing trends in the switching power supply circuits and the DC-DC converter circuits. In addition, the ceramic capacitors, among the key passive components used in these devices, are required to be able to operate under the operating conditions of higher frequency and higher voltage.
Additionally, the ceramic capacitors also have several other issues. For instance, the ceramic capacitor is typically composed of a disc-shape dielectric ceramic element assembly and an electrode is provided on both surfaces in the main area of the disc-shape dielectric ceramic element assembly. Each of the electrodes is connected with a connecting wire (or a lead), and the whole structure is covered by a protective coating or molding material. Silver (Ag) has long been used as the electrode material for ceramic capacitors. The Ag electrode, however, exhibits issues of substantial internal heat-generation and electro-migration. Silver is also extremely expensive.
Instead of Ag, electrodes made of a less expensive metal, such as copper (Cu) or nickel (Ni), is used among some ceramic capacitors. The electrodes made of Cu or other base metals are provided by baking in a neutral or reductive atmosphere to avoid possible oxidation. In order to prevent the dielectric ceramic element assembly from being reduced in the reductive atmosphere, several approaches have been proposed: improving the baking process and adding a certain additive to a composition of the dielectric ceramic element assembly. For instance, one may add an amount of MgO, CuO, CoO, and/or CeO2 to mixtures of SrTiO, PbTiO3, Bi2O3, and/or TiO2. Then, an electrode made mainly of Cu is deposited and baked on the opposing surfaces of the ceramic dielectric element assembly.
However, the reductive atmosphere needs to be strictly controlled, which would lead to reduced productivity. Furthermore, should an oxidized base metal electrode actually occur, it not only affects the production yield rate but also significantly impairs the productivity over the total manufacturing process. This is because it is difficult to detect and identify this kind of defects with a non-destructive inspection process. Also, the addition of an additive to the dielectric ceramic material requires a highly stringent compositional control, which drives up the cost. Additionally, soldering between a lead wire and an electrode often presents a reliability issue.
Several attempts have been made to overcome the above-described drawbacks. For instance, in order to overcome the problems of poor soldering (between metal electrode and connecting wires) and diffusion of electrode/solder metal atoms into the dielectric layer (thereby weakening the wire-electrode bond and degrading dielectric layer performance), Nagashima, et al (U.S. Pat. No. 6,326,052, Dec. 4, 2001) proposed a complex and costly three-layer electrode structure with each layer having different metal compositions. Similarly, the U.S. Pat. No. 6,043,973 (issued on Mar. 27, 2000 to Nagashima, et al.) teaches a ceramic capacitor in which a three-layered electrode is provided by a dry plating method, the first layer of which being Zn. However, these approaches did not address the internal heat-generation problem, yet did significantly increase the production complexity and costs. Even with a highly sophisticated dielectric ceramic composition designed to reduce heat generation (e.g. as disclosed in U.S. Pat. No. 6,777,109, issued to M. Kimoto, et al on Aug. 17, 2004), a significant amount of heat is still generated anyway, particularly when the ceramic capacitor operates under high-frequency and/or high-voltage conditions for an extended period of time.
Thus, it was an object of the present invention to provide a ceramic capacitor electrode composition that exhibits one or more of the following characteristics: (a) reduced or eliminated electrode migration (e.g. diffusion of metal atoms into the dielectric ceramic structure or the interface zone between a lead wire and the metal electrode); (b) no or little metal oxidation of electrode; (c) good compatibility with soldering material and/or connecting wires; (d) simplified electrode compositions; (e) ease of electrode processing; (f) improved compatibility with the dielectric material; and (g) reduced or eliminated heat accumulation in the capacitor during operations.